This invention relates generally to time delay mechanisms and more particularly to the delay associated with the elapsed time of an optical transmission. In greater particularity, the invention pertains to using a compressible fiber optic line to provide a continuously variable delay of an optical transmission.
Time delays have many uses. In both military and commercial sectors, time delay components are used, for example, in phased array antennas, direction finding arrays, and moving target indicators and simulators. Time delay components are useful in applications requiring phase matching or the comparison of waveforms, pulse trains, and electrical or optical signals over the course of time. For example, in an application in which a train of very short-duration optical pulses are transmitted to and returned from a distant mirror and are compared, one may wish to superimpose the returned signals on the original signals to compare the fidelity of the returned pulse or to infer some property from the transmission medium or the conditions existing at the remote location of the mirror. One may accomplish this by using a beam splitter to split the original pulse into two parts, one of which is transmitted to the distant mirror and one which is delayed in time to coincide with the arrival of the returned transmitted signal.
To date a fixed length of optical fiber has been used in some scenarios to accomplish such delays. These fiber optic delay lines consist of a length of optical fiber in which a short duration pulse is launched. The delay is effected by the time it takes the pulse to travel the length of the fiber and return. Since the fibers length is fixed, the time delay is the product of twice the length divided by the index of refraction of the fiber""s material at the-spectral wavelength of the optical signal.
The waveguiding properties of optical fibers arise from their material composition and configuration. Typically, such materials are silica-based, that is, a glass material based on silicon dioxide doped appropriately with heavier elements in the xe2x80x9ccorexe2x80x9d to provide the necessary waveguide properties for this transmission medium. Normally, the fibers are cylindrical with a denser material in the center or xe2x80x9ccorexe2x80x9d region of the fiber.
Viewed from the perspective of geometrical optics, a light wave launched into one end of the fiber will be subject to multiple internal reflections if the launch angle is less than thexe2x80x9ccriticalxe2x80x9d angle determined from Snell""s Law of Refraction. Those rays which then transmit the fiber in a plane containing the central axis of the optical fiber are referred to as meridonal rays, whereas those rays which exceed the critical angle are lost in the xe2x80x9ccladdingxe2x80x9d. The cladding is a material having an index of refraction that is less than that of the core and, thus, forms the boundary for creating the critical angle of refraction and the total internal reflection causing waveguiding.
In prior methods, the use of a fixed length of optical fiber gave only a single time delay associated with the round-trip transit time of an optical signal. If, for a fixed length of optical fiber, the refractive index of the fiber could be changed for a fixed wavelength of light, then because of the dispersive properties of the medium of transmission, the transit time could also be changed. In U.S. Pat. No. 5,530,778, the inventor described a method and apparatus of altering wavelength in the region of a fiber""s anomalous dispersion to effect continuously variable time delays. In the present invention, a mechanical means is used to change the core-cladding index of a fiber in a continuous and predictable manner to achieve desired variable time delays.
A continuously variable fiber-optic delay line is created from a deformable polymeric fiber in which lighter density elements, e.g. hydrogen, boron, carbon or oxygen, are diffused through the outer surface of the fiber. The diffusion of the lighter elements creates a non-light transmissive cladding that is of a refractive index that increases from the surface of the fiber towards its axis. The cladding longitudinally surrounds a light transmissive core and has a maximum refractive index that is less than any refractive index of the core. A single-mode, graded-index polymeric fiber is thereby created. By applying a reversible and controllable pressure to the exterior of the deformable fiber, the refractive index of the fiber can be changed accordingly. This change in refractive index permits one to continuously vary the time delay of a signal transmitted through a fixed length of the fiber.
It is an object of this invention to provide a continuously variable time delay.
A further object of this invention is to provide a continuously variable time delay in the form of a fixed length optical fiber.
Still a further object of the invention is to provide a continuously variable time delay in the form of a fixed length of optical fiber wherein the time delay is caused by the application of pressure upon the outside surface of the fiber.
Other objects, advantages, and new features of this invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanied drawing.